1. Field of the Invention
The present invention relates to a vacuum pump used for a semiconductor manufacturing apparatus. More particularly, the present invention relates to a surface treatment technique for improving the corrosion resistance and heat releasing property of a vacuum pump.
2. Description of the Related Art
Conventionally, the semiconductor manufacturing apparatus has used a vacuum pump to reduce the pressure in a vacuum chamber and to thereby obtain a predetermined degree of vacuum. As the vacuum pump of this type, a kinetic turbo-molecular pump is known. In the turbo-molecular pump, a rotor shaft integral with a rotor is rotatably supported in a pump case, a plurality of stages of rotor blades are provided on the outer wall surface of the rotor, and a plurality of stages of stator blades positioned between the rotor blades are provided on the inner wall surface of the pump case. When the rotor is rotated at a high speed after the pressure in the vacuum chamber has been made a predetermined value, an evacuating operation in which the rotating rotor blades and the fixed stator blades impart momentum to gas molecules colliding with the blades to transfer the gas molecules is performed. By this evacuating operation, the gas molecules sucked from the vacuum chamber into the pump case are exhausted while being compressed, by which the pressure in the vacuum chamber is reduced.
In the dry etching or CVD (chemical vapor deposition) process in the semiconductor manufacturing apparatus, when etching or cleaning utilizing a plasma reaction is performed, a chlorine-based or fluorine-based process gas having high reactivity is introduced into the vacuum chamber. Because this process gas generally has very high metal erodibility, the turbo-molecular pump that sucks the process gas and performs evacuation is required to have high corrosion resistance of various types of components incorporated in the pump case. Of these components, a component rotating at a high speed, such as the rotor, is usually formed of a light alloy such as an aluminum alloy from the viewpoints of high specific strength and reduced weight, but the corrosion resistance of aluminum alloy is insufficient especially to chlorine-based gas. Conventionally, therefore, plating of the aluminum alloy with a metal having high corrosion resistance, such as a nickel alloy, has widely been performed.
On the other hand, in the turbo-molecular pump of this type, the sucked gas molecules collide with the rotor blades and the stator blades and are compressed, and by frictional heat at the time of collision and compression heat at the time of compression, a rotating body consisting of the rotor and the rotor blades is heated to a high temperature. Also, the rated rotational speed of the rotating body is generally as high as 20,000 to 50,000 rpm, so that the rotating body is subjected to a great tensile stress due to a centrifugal force. Therefore, if the operation is continued for a long period of time, the rotating body in a state of being heated and subjected to tensile stress is plastically deformed gradually, causing creep deformation, and hence comes into contact with a fixed-side component facing to the rotating body with a minute gap provided therebetween. Thus, a crack is created at a part of the rotating body by this contact, and stress concentrates there, which may result in a breakage of the rotating body.
The principal reason why the rotating body is broken in the turbo-molecular pump is thought to be the overheating of the rotating body at the time of high-speed operation. Therefore, in order to prevent the breakage of rotating body, it is necessary to efficiently release heat accumulated in the rotating body to perform cooling. The method for cooling is broadly divided into conduction heat release and radiation heat release. As an example of the former conduction heat release, a method in which heat conduction is performed through a bearing and a method in which heat conduction is performed through a gas are known. Also, as an example of the latter radiation heat release, a method in which the heat of rotor is radiated to a component on the fixed side is known.
However, in the case of the former conduction heat release utilizing a bearing, for example, if the rotor is supported by a magnetic levitation bearing, since the rotor shaft and the bearing are not in contact with each other, it is impossible to directly conduct the heat of rotor from the rotor shaft to the bearing. Also, in the case of the conduction heat release utilizing a gas, when a gas having low heat conductivity of gas molecule, such as argon, krypton, xenon, and other rare gases, is exhausted, heat conduction through the gas is scarcely anticipated. It can be thought that heat conduction is performed by filling the pump case with a purge gas with high heat conductivity, such as hydrogen or helium. In this case, since a large amount of gas flows in the pump case, the pressure in the pump case or the vacuum chamber fluctuates greatly, so that the quantity of heat capable of being released is restricted.
Thereupon, the rotating body is cooled by the latter radiation heat release. At this time, if the rotor is subjected to nickel alloy plating as described above, the quantity of heat radiated from the surface of rotor is decreased, and therefore the heat releasing property is decreased remarkably. The reason for this is that the emissivity of nickel is about 0.1 to 0.2 while the emissivity of aluminum as material of the rotor is about 0.3, so that the emissivity of the rotor as a whole is decreased by nickel alloy plating.
The emissivity is defined as the ratio of the luminance of heat radiation on an object to the luminance of heat radiation on a black body having the same temperature, in other words, the ratio of the quantity of radiated heat on an object to that on a black body having the largest quantity of radiated heat, which is represented with the black body being 1. As an object comes closer to black color, the emissivity increases, and the quantity of heat radiated from the surface thereof increases. That is to say, if the rotor made of an aluminum alloy is subjected to nickel alloy plating to improve corrosion resistance to corrosive gas, the quantity of heat radiated from the rotor surface decreases, and hence radiation transmission to the fixed side becomes difficult to perform, which results in a disadvantage that the rotating body cannot be cooled efficiently.
Japanese Patent Laid-Open No. 11-257276 has disclosed a technique for applying a metal plating layer containing ceramic particles onto the surface of the rotor made of an aluminum alloy. According to this technique, it is thought that the quantity of heat radiated from the surface thereof increases because the emissivity of ceramic particles is about 0.7 to 0.8. However, the ceramic particles are dispersed in the nickel alloy, and the quantity of heat radiated from the nickel alloy occupying most of the surface area is still small. Therefore, the emissivity of the whole of the surface of metal plating layer is not so high, and it cannot be said that the heat releasing property of rotor is sufficient. To solve this problem, it can be thought that the content of ceramic particles is increased. In this case, however, the bonding strength of nickel alloy that joins ceramic particles becomes low, so that the ceramic particles may undesirably be peeled off from the metal plating layer by a centrifugal force during high-speed rotation.
Japanese Patent Laid-Open No. 2001-193686 has disclosed a technique for improving the emissivity of a component surface by providing a coating layer in which particulates of ceramic or resin etc. are added to a black nickel alloy or a black chromium alloy on the surface of a component in the vacuum pump. Also, it is common practice to form a ceramic layer on the surface of a component by thermal spraying or to form a layer on the surface of a component by the coating, bonding, etc. of a mixture of ceramics with a binding agent such as a polymer. With such methods, however, the polymer used as an additive or a binding agent has corrosion resistance lower than that of the nickel alloy layer, which presents a problem in that corrosion proceeds from that portion, and attacks the base material. Also, since only a porous layer is obtained by thermal spraying, there arises a possible problem in that the corrosive gas intrudes into the base material through the pores to corrode the base material.